This invention relates to an electronic musical instrument for realizing synthesis of a tone according to a fixed formant.
Natural musical instruments are known to have their own fixed formants peculiar to structures of the musical instruments such as a configuration of a sound-board in the case of a piano. A fixed formant exists in a human voice also and this fixed formant characterizes a tone color peculiar to a human voice. In order to simulate a tone color of a natural musical instrument or a human voice in an electronic musical instrument, a musical tone must be synthesized in accordance with a fixed formant peculiar to the tone color.
Several methods have been proposed for realizing a fixed formant in an electronic musical instrument. One is to use an analog type fixed filter. This method, however, is disadvantageous in that an analog type fixed filter having a sufficient Q(sharpness) for realizing a fixed formant is extremely expensive and that a fixed filter must be provided for each fixed formant or each tone color.
There are the following prior art digital type electronic musical instruments capable of synthesizing a tone on the basis of a fixed formant:
a. U.S. Pat. No. 3,809,786 entitled "Computer Organ;" PA0 b. Japanese patent preliminary publication No. 1979-81824 entitled "Electronic Musical Instrument;" PA0 c. U.S. Pat. No. 4,018,121 entitled "Method of Synthesizing a Musical Sound;" PA0 d. Japanese patent preliminary publication No. 1980-18623 entitled "Electronic Musical Instrument".
The above listed U.S. Pat. No. 3,809,786 and Japanese patent preliminary publication No. 1979-81824 disclose the art of producing a partial tone signal for each harmonic component of a tone, multiplying it with an amplitude coefficient set for each harmonic and subsequently adding partial tone signals together to produce a tone signal of a desired spectrum structure. Such type of electronic musical instrument is convenient for synthesis of a tone in which the spectrum structure of respective harmonic components is not affected by change of its fundamental frequency, but is not suited for synthesis of a tone made on the basis of a fixed formant. For realizing a fixed formant is such type of electronic musical instrument, a set of harmonic amplitude coefficients which are different one key from another must be provided for each fixed formant. This requires a memory of a very large capacity.
The above mentioned U.S. Pat. No. 4,018,121 discloses the art of synthesizing a tone of a desired spectrum structure by a frequency modulation computation in an audio range. The Japanese patent preliminary publication No. 1980-18623 discloses the art of synthesizing a tone on the basis of a fixed formant by utilizing the frequency modulation computation.
According to the Japanese patent preliminary publication No. 1980-18623, a center frequency of a fixed formant is modified to a harmonic frequency which is nearest to the center frequency among harmonic frequencies of a tone designated by depression of a key and a formant having the modified center frequency (i.e. harmonic frequency) as its central component is synthesized by a frequency modulation computation. The reason for modifying the center frequency of the fixed formant to the nearest harmonic frequency is that side frequencies obtained by conforming a carrier frequency and a modulating frequency for the frequency modulation computation to a harmonic frequency of a desired tone constitute harmonic components of the tone. If, however, there is discrepancy between the formant center frequency and the harmonci frequency, the synthesized formant is somewhat different from a desired fixed formant. This difference poses a problem particularly when the fundamental frequency (f.sub.0) of a tone to be generated is relatively high. An example of a spectrum envelope in a case where the fundamental frequency (f.sub.0) is low is shown in FIG. 1(a) and an example of a spectrum envelope in a case where the fundamental frequency (f.sub.0) is high is shown in FIG. 1(b). In these figures, solid lines designate spectrum envelopes of fixed formants to be synthesized and broken lines those of formants which are actually produced by the prior art method. In the case where the fundamental frequency (f.sub.0) is low, the interval between respective harmonic frequencies (f.sub.0, 2f.sub.0, 3f.sub.0 . . . ) is relatively narrow and accordingly, difference between center frequencies (f.sub.f1, f.sub.f2) of a desired fixed formant and harmonic frequencies (3f.sub.0, 8f.sub.0) in the vicinity of the center frequencies is not so large, as shown in FIG. 1(a), and difference between a formant synthesized about the harmonic frequencies (3f.sub.0, 8f.sub.0) and the desired fixed formant is of a negligible amount. In the case where the fundamental frequency (f.sub.0) is high, however, the interval between harmonic frequencies (f.sub.0, 2f.sub.0, 3f.sub.0 . . . ) is wide and difference between the center frequencies (f.sub.f1, f.sub.f2) of the desired fixed formant and the harmonic frequencies (f.sub.0, 2f.sub.0) in the vicinity thereof is also wide. In this latter case, a formant synthesized about the harmonic frequencies (f.sub.0, 2f.sub.0) is substantially distorted from the desired fixed formant as shown in FIG. 1(b) with a result that the tone color of the tone is adversely affected. For example, an original level of the harmonic frequency 2f.sub.0 in FIG. 1(b) is l.sub.0 but an actual level thereof is L which is much higher than l.sub.0 due to the shift of the formant as shown by the broken line. As a result, it is difficult to obtain a desired tone color.